This invention relates to trapping assemblies and to valve means suitable for use with such assemblies as part of the process for introducing trapped solutes into instruments such as gas, liquid or supercritical fluid chromatographs that are used to measure amounts of such solutes.
The use of gas, liquid and supercritical fluid chromatographs (hereinafter collectively referred to as "chromatographs") have long been used to measure amounts of solutes in fluids. In a gas chromatograph the carrier fluid is gas, e.g., nitrogen, and in a liquid chromatograph it is a liquid, e.g., methyl alcohol. In SFC chromatographs the carrier fluid is ordinarily a gas, e.g., CO.sub.2, which is densified with increased pressure above its critical point. The density and thus the effective solvent power of a supercritical fluid can be controlled by pressure.
It has become increasingly important to be able to measure very small, even trace, amounts of solutes in a carrier fluid. This is particularly desirable in measuring very small amounts of contaminants, such as organic chemicals, pesticides, etc., in drinking water or foods in amounts of the order of 1 ppb. In such circumstances, the amount of solute may be below the minimum detectable quantity (MDQ) for most chromatographs.
In the method and apparatus of Poole et al., U.S. Pat. No. 4,500,432, granted Feb. 19, 1985 there is provided a way to concentrate solutes contained in fluids before they are applied to chromatographs for analysis. In general, the technique of Poole et al. involves concentrating a solute by passing a solvent containing it through a first trapping means (e.g., a packed column) that adsorbs the solute and passes the solvent to waste, passing a fluid (e.g., a supercritical fluid) through the first trapping means to dissolve or solubilize the solute therefrom and carry it into a second trapping means, and reducing the solubility parameter of the fluid in the second trapping means. Where a supercritical fluid carries the solute, the last step can involve passing the fluid from a high pressure to a much lower pressure. This permits the fluid to escape from the second trapping means leaving the solute concentrated therein. The second trapping means can be used by itself when a vessel containing a range of materials (solids, semi-solids, liquids dispersed as a stationary phase) replaces the first trapping means.
Currently known approaches to achieve pressure drops from high pressure systems to lower pressures include: (a) static orifices which are typically holes of about 3 to 20 microns in diameter in thin metal foil or at the end of converging ducts, or (b) lengths of capillary tubing (e.g., 20 to 50 microns ID). Neither of the foregoing decouple control of pressure (and therefore density) from linear flow rate.
Often in larger systems (e.g., small pilot plant scale), conventional needle valves are used for pressure drop. These tend to suffer from large inaccessible volumes (dead volume) and inappropriately placed boundaries to the expanding stream so that sampling of high pressure fluid is not representative. Large valves also tend to suffer from poor design in getting sufficient heat into the device to balance heat lost during expansion and, thus, tend to "ice up" causing erratic flow or stoppages. Available back pressure regulators, using manually set, springdriven control pistons, do not contain nozzle geometry for sampling with the capability of electronic control of the pressure.